The present invention relates to an optical head for an optical recording and reading medium, and in particular, to devising a thinner optical head.
Optical heads are important structural components for reading signals from optical recording media such as compact disks (CDs) or digital video disks (DVDs). Not only signal detection functions but also control mechanisms such as focus servos or tracking servos are necessary for optical heads to read out a signal from an optical recording medium.
FIG. 24 illustrates a typical conventional optical head. As is shown in this drawing, a collimator lens 3 collimates a laser beam 2, which is emitted from a semiconductor laser 1 serving as the light source, into parallel light. After the laser beam 2 has passed through a focus/track error signal detection element 8, its optical axis is deflected 90xc2x0 by a mirror 20 and the light enters an objective lens 4, which focuses the laser beam 2 on an optical disk 11. The laser beam is reflected and returns on the same light path. It is turned into parallel light by the objective lens 4, then reflected by the mirror 20, and enters the focus/track error signal detection element 8. When the laser beam 2 enters the focus/track error signal detection element 8, it is divided into two beams, which are focused on the photo-detectors 13a and 13b. Thus, regeneration signals and servo signals, i.e. focus error signals and track error signals, can be read.
As can be seen from FIG. 24, the height of the optical head can be expressed as the sum of the working distance (WD), the thickness of the objective lens 4, the space between a lower portion of the objective lens 4 and an upper portion of the mirror 20 (referred to as xe2x80x9clens-mirror spacexe2x80x9d in the following), and the height lz of the mirror 20.
When trying to devise a thinner optical head, the minimum total length of WD, lens thickness and the lens-mirror space are for the most part governed by the type of the optical disk 11. For example, for DVDs, the minimum values for WD, lens thickness and lens-mirror space can be estimated at 1.1 mm each, but the height lz of the mirror 20 has to be larger than a beam diameter w1, for example 3 mm. Consequently, in this case, the minimum height of the optical head can be estimated at 6.3 mm, and it is difficult to devise a thinner head.
It is a purpose of the present invention to solve these problems of the prior art and to provide a thinner optical head.
To achieve this purpose, an optical head according to a first configuration of the present invention comprises a first grating element and a second grating element arranged in that order in a light path between a light source and an objective lens; and a light path alteration member arranged in a light path between the light source and the first grating element or in a light path between the second grating element and the objective lens. In such an optical head, the optical distance between the first grating element and the second grating element can be reduced, so that a lateral shift of the optical axis due to wavelength variations in the light source can be reduced. As a result, the lateral shift of the optical axis from the center of the objective lens can be reduced and a focussing spot with a favorable circular shape can be formed.
In the optical head according to the first configuration of the present invention, it is preferable that the light path alteration member is arranged in the light path between the light source and the first grating element; light emitted from the light source passes through the light path alteration member and then enters the first grating element; light diffracted by the first grating element enters the second grating element; and light diffracted by the second grating element enters the objective lens and is focused on a recording medium. In such a preferable example, light can be deflected toward the objective lens using the second grating element, so that a thinner optical head can be realized.
It is also preferable that the first grating element is a reflection element; the light path alteration member is a first transparent substrate having a first surface; the first grating element and the light path alteration member are arranged so that an angle defined by the optical axis of light emitted from the light source and a normal on the first surface is at least the critical angle, and an angle defined by the normal on the first surface and the optical axis of light diffracted by said first grating element to the second grating element is smaller than the critical angle; light emitted from the light source is reflected from the first surface and enters the first grating element; and light reflected and diffracted by the first grating element passes the first surface and enters the second grating element. In such a preferable example, the optical axis of the light emitted from the light source can be shifted toward the recording medium, so that a thinner optical head can be achieved. In this case, it is even more preferable that the first transparent substrate is a triangular prism having a slanted face, a bottom face and a side face; the slanted face is the first surface; the first grating element is provided on the bottom face; and light emitted from the light source enters the first transparent substrate through the side face. In this specification, a xe2x80x9cslanted surfacexe2x80x9d means a surface that is not substantially perpendicular or parallel with respect to the direction of the light beam. Moreover, in this case, it is preferable that the second grating element is a transmission element; the optical head further comprises a second transparent substrate, on an upper face of which the second grating element is formed; a multi-layered film is formed on the first surface of the first transparent substrate; and the first transparent substrate and the second transparent substrate are integrated into one component by the multi-layered film. It is also preferable that the second grating element is a transmission element; the optical head further comprises a second transparent substrate, on an upper face of which the second grating element is formed; and an air gap is provided between the first transparent substrate and the second transparent substrate. It is also preferable that the second grating element is a transmission element; the optical head further comprises a second transparent substrate on an upper face of which the second grating element is formed; and the second transparent substrate is a triangular prism. It is also preferable that the second grating element is a reflection element; the optical head further comprises a second transparent substrate on a lower face of which the second grating element is formed; a multi-layered film is formed on the first surface of the first transparent substrate; the first transparent substrate and the second transparent substrate are integrated into one component by the multi-layered film; and the first grating element and the second grating element are arranged on the same plane. In these preferable examples, the first grating element and the second grating element can be easily manufactured.
It is preferable that a first incidence angle defined by the optical axis of a laser beam travelling from the light path alteration member to the first grating element and the normal on the first grating element is larger than an outgoing angle defined by the optical axis of diffracted light from the first grating element and the normal on the first grating element, and a second incidence angle defined by the optical axis of the laser beam from the first grating element entering the second grating element and the normal on the second grating element is larger than an outgoing angle defined by the normal on the second grating element and the optical axis of the light diffracted by the second grating element. In this preferable example, beam formation is performed and the light utilization efficiency can be raised. In this case it is even more preferable that the outgoing angles of light diffracted by the first grating element and the second grating element are substantially 0xc2x0. In this preferable example, beam formation can be performed with very high efficiency. It is also preferable that the optical head further comprises a first transparent substrate and a second transparent substrate, the second grating element being formed on the upper face or the lower face of the second transparent substrate; wherein the first transparent substrate and the second transparent substrate are integrated into one component by the first grating element. It is also preferable that the first incidence angle and the second incidence angle are 45xc2x0 to 60xc2x0. In these preferable examples, effective beam formation can be performed and the light utilization efficiency can be raised.
In an optical head according to the first configuration of the present invention, it is preferable that an optical axis change of the light diffracted by the first grating element due to wavelength variation is at least partially cancelled out by an optical axis change of the beam diffracted by the second grating element. In this preferable example, an inclination of the optical axes due to wavelength variations caused by a change in the operation temperature of a semiconductor laser light source can be prevented.
Moreover, in an optical head according to the first configuration of the present invention, it is preferable that the first grating element and the second grating element are linear grating elements with the same uniform grating period. In this preferable example, the influence of wavelength variations can be completely eliminated.
Moreover, in an optical head according to the first configuration of the present invention, it is preferable that the first grating element and the second grating element are volume holograms having a periodic refractive index distribution. In this preferable example, a high diffraction efficiency of at least 90% can be realized even when the diffraction angle is large (for example, 45xc2x0). In this case, it is also preferable that the polarized light entering the volume holograms is S-polarized light on both the outgoing light path and the return light path. In this preferable example, the volume holograms can be easily manufactured, and the light utilization efficiency can be raised. Moreover, in this case, it is preferable that the optical head further comprises a polarizing focus/track error signal detection element, wherein the amplitude of spatial modulation of the refractive index in the volume holograms is adjusted so that the product of a first-order diffraction efficiency of S-polarized light and a first-order diffraction efficiency of P-polarized light is maximized. In this preferable example, the total light utilization efficiency can be raised.
Moreover, in an optical head according to the first configuration of the present invention, it is preferable that the optical head further comprises a polarizing focus/track error signal detection element; and a xc2xc wavelength plate arranged in a light path between the second grating element and the objective lens. In this preferable example, the light beam passes the polarizing focus/track error signal detection element on the outgoing light path almost without loss, because the S-polarized light is provided as light coming from the light source. Because the beam passes the xc2xc wavelength plate on the outgoing and the return way, the light entering the focus/track error signal detection element is P-polarized light on the return way, so that it can be effectively diffractive toward the photo-detectors.
Moreover, it is preferable that the diffraction angles of the first grating element and the second grating element are at least 45xc2x0. In this preferable example, the optical head can be made extremely thin.
Moreover, in an optical head according to the first configuration of the present invention, it is preferable that the first grating element and the second grating element are provided on the same transparent substrate. It is also preferable that the first grating element and the second grating element are provided on the same plane on the same transparent substrate. It is also preferable that the first grating element and the second grating element are provided on the same surface of the transparent substrate. It is also preferable that a triangular prism is arranged on the transparent substrate so that its lower face opposes the first grating element, and a slanted face of the triangular prism is the light path alteration member. It is also preferable that a reflection plate is arranged on a back face side of the transparent substrate, separated therefrom by an air layer; and light diffracted from the transparent substrate into the air layer is reflected from the reflection plate and enters the second grating element. In this preferable example, the light passing the first grating element and entering the transparent substrate is refracted at the border between the transparent substrate and the air layer, reflected from the reflection plate, and enters the second grating element, so that the sum of the thicknesses of the transparent substrate, the air layer, and the reflection plate can be reduced. As a result, a thinner optical head can be achieved.
Moreover, in an optical head according to the first configuration of the present invention, it is preferable that the second grating element converts parallel light into divergent light and divergent light into parallel light. In this preferable example, a two-wavelength configuration using one collimator lens and one objective lens can focus light of wavelengths corresponding to optical disks with a relatively thick protective layer such as CDs and optical disks with a relatively thin protective layer such as DVDs favorably and without aberration on the pit surfaces of the disks.
Moreover, in an optical head according to the first configuration of the present invention, it is preferable that the first grating element and the second grating element comprise multi-layered volume holograms, each layer corresponding to a different wavelength. In this preferable example, several kinds of disks can be used with corresponding different wavelengths.
Moreover, in this case, it is preferable that the thickness of each layer in the multi-layered volume hologram corresponds to a different wavelength. In this preferable example, the tolerances for the diffraction efficiencies of the different wavelengths can be set to an optimum for each kind of disk.
In this case, it is even more preferable that the thickness of each layer in the multi-layered volume hologram is substantially proportional to different a wavelength.
It is also preferable that each layer in the multi-layered volume hologram has fringes with periodic refractive index distributions and different inclination angles. In this preferable example, the generation of unnecessary diffractive light in a layer that does not correspond to that wavelength can be reduced and the light utilization efficiency increased.
It is also preferable that the multi-layered volume holograms have fringes with periodic refractive index distributions and identical inclination angles. In this preferable example, the inclinations of the optical axes at the wavelengths of the diffracted light travelling from the first grating element to the second grating element can be equalized.
Moreover, it is also preferable that the first grating element and the second grating element comprise two-layered volume holograms, each of the two layers corresponding to one of the two wavelengths xcex1 and xcex2 characterized by 0.60 xcexcmxe2x89xa6xcex1xe2x89xa60.68 xcexcm and 0.76 xcexcmxe2x89xa6xcexc2xe2x89xa60.87 xcexcm. In this preferable embodiment, the unnecessary diffraction of light of other wavelengths at the two-layered first grating element and the two-layered second grating element can be reduced, and, for example, DVDs, CD-Rs, and CDs can be favorably read.
It is also preferable that the first grating element and the second grating element comprise three-layered volume holograms, each of the three layers corresponding to one of the three wavelengths xcex1, xcex2 and xcex3 characterized by 0.38 xcexcmxe2x89xa6xcex1xe2x89xa60.52 xcexcm, 0.60 xcexcmxe2x89xa6xcex2xe2x89xa60.68 xcexcm and 0.76 xcexcmxe2x89xa6xcex3xe2x89xa60.87 m. In this preferable embodiment, the unnecessary diffraction of light of other wavelengths at the three-layered first grating element and the three-layered second grating element can be reduced, and, for example, many optical disks such as high-density disks with more than 10 GByte capacity, DVDs, CD-Rs, and CDs can be favorably read.
An optical head according to a second configuration of the present invention comprises a light source emitting a light beam in a first direction; first deflector for deflecting the light beam from the first direction into a second direction; second deflector for deflecting the light beam deflected by the first deflector into a third direction; and an objective lens for focusing the light beam deflected by the second deflector onto an optical recording medium; wherein the third direction is substantially perpendicular to the recording surface of the optical recording medium, and the length of the second deflector in the third direction is smaller than the width in the third direction of the light beam travelling in the first direction. According to this second configuration of the optical head, a light beam emitted from the light source can be deflected in a diagonal direction (the second direction) by the first deflector, so that the incidence angle into the second deflector can be tilted with respect to the y-axis direction. As a result, the height of the second deflector, which conflicts with the objective lens, can be reduced, so that a thinner optical head can be achieved.
In this second configuration of the optical head, it is also preferable that the first deflector is a triangular prism.
In this second configuration of the optical head, it is also preferable that the second deflector is a reflection grating element.
In this second configuration of the optical head, it is also preferable that the second deflector is a mirror.